1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly to an apparatus for reading an original image optically and converting it to digital image data, and then forming an image on a sheet in an electrophotographic method according to the image data.
2. Description of Related Art
Recently, in a field of electrophotographic copying apparatus, image processing is digitized for carrying out multifunctional image edit and full color image formation. In such digitization, a print head of a laser printer is used as image writing means for writing an image on a photosensitive member. In this print head, a laser diode is modulated according to image data, and a laser beam which is emitted from the laser diode is deflected by a polygonal mirror and is scanned on the photosensitive member line by line. This is called main scanning. Sub scanning is carried out by rotation of the photosensitive member. An image is printed on the photosensitive drum two-dimensionally by the main and the sub scanning. In this print head (a laser beam optical scanning apparatus), dot-by-dot modulation can be done easily, and a line in the main scanning direction can be formed to have less irregularity. Also, the beam diameter can be easily changed. Additionally, the print head can be composed at a low cost.
On the other hand, a print head of a digital copying machine requires high performances compared to the laser printer. One of the required performances is accuracy of a printing position. In the digital copying machine, since an error probably occurs in an image reading unit (an image scanner unit), performances such as magnification in the main scanning direction and uniformity of pitch in the main scanning direction are required to be higher than those of the laser printer. High accuracy of a printing position in the sub scanning direction is also required. Irregularity of the printing position appears as periodical unevenness of density (nonuniformity of pitch). This irregularity is caused by periodical movement of the printing position in the sub scanning direction, which results from an error in perpendicularity of deflecting facets of the polygonal mirror and vibration of the polygonal mirror. A tolerance of irregularity in the printing position in the sub scanning direction is smaller in the digital copying machine compared to the laser printer.
An area gradation method, a density gradation method or a many-valued dither method which is the middle of the above two methods can be used to express a half tone. In the area gradation method, several dots are considered as one unit, and the gradation is expressed by changing the number of printed dots in the unit to change a written area. In the density gradation method, the gradation is expressed by changing the density of every single dot. The density gradation method surpasses the area gradation method in expressing the half tone. However, the density gradation method is sensitive to the irregularity of the printing position in the sub scanning direction, and the tolerance of the irregularity is small. In the area gradation method, if the correction factor is about one-several decades, the irregularity of the printing position in the sub scanning direction becomes about several .mu.m which is no problem. However, in the density gradation method, the accuracy which is 1 .mu.m or less is required. Therefore, the perpendicularity of the deflecting facets of the polygonal mirror is needed to be improved or an optical system of high correcting performance is needed to be used.
Improving the accuracy in the perpendicularity of the deflecting facets of the polygonal mirror raises the manufacturing cost of the polygonal mirror itself. Thus, an optical system which can correct the perpendicularity of the polygonal scanner effectively is needed to be used. In an optical system for correcting the perpendicularity of the deflecting facets of the polygonal mirror, an area around a deflecting point and a scanned surface (a photosensitive member) are conjugate in the sub scanning direction, and the smaller the magnification is, the more effective the correction is. Therefore, an optical system which has small magnification in the sub scanning direction is needed to be used. Assumed that the focal distance of a f .theta. lens is fixed, the diameter of a lens near the scanned surface must be larger, in order to reduce the magnification in the sub scanning direction.
On the other hand, in the print head of the digital copying machine, the beam diameter is preferably small in order to improve the tone expression. Also, curvature of field and distortion needs to be small. Thereby, more lenses are necessary, and this causes a raise of the cost. In order to solve this problem, it is possible that aspherical lenses are used to decrease the number of necessary lenses, using plastic, not glass, as the material. However, plastic may change its shape and its reflection factor with a change of temperature, and this causes a change of a focal distance. The change of the focal distance appears as a change of magnification or blooming (a change of the beam diameter).
In order to adjust the blooming caused by the change of the focal distance, the following way is possible: the laser beam is lead into a slit and is detected by a photoelectric transfer element at position optically equivalent to the photosensitive member; and the focal distance is adjusted based on the detected beam diameter. However, if the slit is formed extending parallel with the main scanning direction, the photoelectric transfer element can not detect the beam diameter in the main scanning direction. Moreover, if the incident beam is shifted even in a small degree in the sub scanning direction, the detection of the beam diameter becomes impossible. Also, since focusing is conducted separately in the main scanning direction and the sub scanning direction, the blooming may occur in the sub scanning direction while the focus in the main scanning direction is adjusted.